package org.apache.lucene.store;

/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import java.io.IOException;
import java.util.zip.CRC32;
import java.util.zip.Checksum;

/** Writes bytes through to a primary IndexOutput, computing
 *  checksum.  Note that you cannot use seek().
 *
 * @lucene.internal
 */
public class ChecksumIndexOutput extends IndexOutput {
    IndexOutput main;
    Checksum digest;

    public ChecksumIndexOutput(IndexOutput main) {
        this.main = main;
        digest = new CRC32();
    }

    @Override
    public void writeByte(byte b) throws IOException {
        digest.update(b);
        main.writeByte(b);
    }

    @Override
    public void writeBytes(byte[] b, int offset, int length) throws IOException {
        digest.update(b, offset, length);
        main.writeBytes(b, offset, length);
    }

    public long getChecksum() {
        return digest.getValue();
    }

    @Override
    public void flush() throws IOException {
        main.flush();
    }

    @Override
    public void close() throws IOException {
        main.close();
    }

    @Override
    public long getFilePointer() {
        return main.getFilePointer();
    }

    @Override
    public void seek(long pos) {
        throw new RuntimeException("not allowed");
    }

    /**
     * Starts but does not complete the commit of this file (=
     * writing of the final checksum at the end).  After this
     * is called must call {@link #finishCommit} and the
     * {@link #close} to complete the commit.
     */
    public void prepareCommit() throws IOException {
        final long checksum = getChecksum();
        // Intentionally write a mismatched checksum.  This is
        // because we want to 1) test, as best we can, that we
        // are able to write a long to the file, but 2) not
        // actually "commit" the file yet.  This (prepare
        // commit) is phase 1 of a two-phase commit.
        final long pos = main.getFilePointer();
        main.writeLong(checksum - 1);
        main.flush();
        main.seek(pos);
    }

    /** See {@link #prepareCommit} */
    public void finishCommit() throws IOException {
        main.writeLong(getChecksum());
    }

    @Override
    public long length() throws IOException {
        return main.length();
    }
}
